Most maps can be classified into two main groupings: general purpose and thematic. General purpose maps are often used for reference purposes and can exhibit a variety of information including physical land features and political boundaries. Examples of general purpose maps include those found in a standard geographic atlas, or road maps. Thematic (or special-purpose) maps are typically used to convey a specific theme to a particular audience. A map of the United States in which the color of a state represents its census population, is an example of a thematic map.
There are types of maps which do not fall into either of the two main categories, or which exhibit properties of both. Since maps can display multiple levels of information, the separation between different types of maps is not always clear. In addition, maps have uses in varied fields, and as such, sometimes terminology may seem conflicting. However, the key purpose of this article is to introduce you to the most commonly used terms in an effort to help you search for new map data to use with Dundas Map.
General Purpose Maps
The most common type of general purpose map is a topographic map. Topographic maps are often used as reference maps, and typically display both natural land features (such as coastlines and bodies of water) as well as political boundaries. Topographic maps also display elevation (height above sea level), using either coloring (relief shading) or contour lines. The United States Geological Survey (USGS) produces topographic maps in a series, and at standard map scales (such as 1:24,000).
Planimetric maps are two-dimensional maps which are similar to topographic maps, but do not show any elevation. Planimetric maps tend to display natural features such as lakes and rivers, or man-made features such as roads and city boundaries. These types of maps can serve as the basis for cadastral maps which document the boundaries and ownership of parcels of land.
Base maps act as a foundation for superimposing additional layers of information. For example, a thematic map can be constructed from a base map using a specific colorization scheme. A base map typically contains some natural features such as coastlines, and some man-made features such as political boundaries. The map library distributed with Dundas Map consists of base maps representing various regions of the world. Planimetric maps are often used as base maps.
|Figure 1: Base map of the world from the Dundas map library.|
A thematic map shows how qualitative and quantitative data are distributed geographically. Thematic maps usually build on top of a base map in order to convey a specific geographic theme, such as population by state, or sales per region.
Qualitative Thematic Maps
|Figure 2: A geographical region map (shown left), and a resource map (shown right).|
Quantitative Thematic Maps
Quantitative thematic maps are also known as statistical maps and use a visual mechanism, such as color, to indicate the quantity of a data attribute at different locations on a map. Examples of this type of map are discussed below.
Choropleth maps use a uniform color or pattern to fill a geographical shape on a map according to the quantity of a data attribute associated with that shape. For example, a choropleth population map of the United States might assign the color green to all states with a population of between 10 and 20 million people.
Isopleth maps also use color, except that the boundaries between color areas are defined by isolines representing points with equal data attribute values. A typical weather or temperature map is an example of an isopleth map. Isopleth maps are ideal when your data values vary continuously and smoothly over space.
Proportional symbol maps use scaled symbols or icons in order to indicate the relative quantity of a particular data attribute. A larger symbol, for example, indicates a larger data value for a location on the map. Other techniques involve the use of symbols such as bar or pie indicators in which the actual size of each symbol is fixed, but the symbol appearance varies proportionally with the data attribute value. For example, given a larger data value, a larger slice of a pie indicator would be drawn.
Dot (or dot density) maps use a fixed size dot symbol on a map in order to represent a fixed quantity of data. For example, a single dot symbol on a population map could represent one million people. If you then view a map of the United States which is constructed using such symbols, you will see that areas of high dot density indicate regions of greater population while low dot density areas indicate sparsely populated regions.
|Figure 3: Proportional symbol map using bar indicators to represent population ranges.|